Nitrogen oxides (referred to hereinafter as NO.sub.x) which are contained in various types of exhaust gases not only threaten the health of individuals but are in general environmentally undesirable since they may cause acid rain or photochemical smog. Accordingly, the development of an effective means for exhaust gas treatment, specifically the development of an effective NO.sub.x abatement method, has been sought by those in the art.
Some processes for reducing the NO.sub.x content of exhaust gases using catalysts have been put into practice.
As examples of such processes, there can be mentioned a ternary catalyst process for gas fueled automobiles and a selective catalytic reduction process using ammonia for exhaust gases which are discharged from heavy industrial installations comprising, for example, boilers.
As another example of such a process, there can be mentioned a process for removing NO.sub.x from exhaust gases using hydrocarbons, which comprises bringing a gas containing NO.sub.x into contact with a metal oxide catalyst, for example, a catalyst composed of a metal such as copper supported on alumina in the presence of a hydrocarbon (see, JP-A-63-100919 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")).
In the first process proposed above, i.e., in the ternary catalyst process, the hydrocarbon components and carbon monoxide initially present in the waste gas which is exhausted upon combustion of the fuel are converted to water and carbon dioxide with the aid of a catalyst while consuming the oxygen in the NO.sub.x to thereby reduce it to nitrogen. In this process, however, the combustion must be carefully controlled in that a stoichiometric amount of oxygen is supplied to the combustion system so that the oxygen content in the NO.sub.x compensates for the amount of oxygen consumed in the oxidation of the hydrocarbon components and carbon monoxide. Thus, this process is, in principle, not applicable to a combustion system where excess oxygen is present, such as a system involving a diesel engine.
The second process, i.e., the selective catalytic reduction process using ammonia, is hard to handle and requires a very large scale installation for the sake of safety, since the process uses ammonia which is very toxic and must generally be handled under high pressure. Accordingly, there is difficulty in applying this process to systems comprising a small scale exhaust gas source, particularly movable exhaust gas sources, from the technological viewpoint. Further, the process is disadvantageous from the economical viewpoint.
The last process mentioned above, i.e., the process for removing NO.sub.x from exhaust gases using a metal oxide catalyst, is intended for application mainly to gas-fueled automobiles. Accordingly, the process is unsuitable for treating exhaust produced by combustion in a diesel engine. If the process were to be applied to an exhaust gas from a diesel engine, the activity of the catalyst would be insufficient to achieve the desired level of NO.sub.x removal. In more detail, the catalyst composed of a metal such as copper supported on alumina not only suffers degradation due to sulfur oxides, discharged from the diesel engine, but also loses activity due to aggregation or like behavior of the metal component thereof. Thus, this process has not been put into practice for the removal of NO.sub.x from exhaust gases discharged from diesel engines.
The present inventors previously proposed processes using a catalyst obtained by treating a metal oxide with a compound having a sulfate group to solve the problems mentioned above. The present inventors have further made studies and found that when catalysts comprising a polyvalent metal phosphate, a polyvalent metal sulfate or a metal aluminate are used, the NO.sub.x components can be effectively removed from exhaust gases even in the presence of sulfur oxides without a loss of activity. The present invention was completed on the basis of this finding.